1. Field of the Invention
This invention concerns a copper alloy for use in electric and electronic parts used, for example, in semiconductor lead frames, terminals, connectors and bus bars and, more in particular, it relates to a copper alloy available at a reduced cost and having a conductivity of 50% IACS or more while having high strength substantially comparable with that of 42 alloy, as well as having softening resistance, favorable shearing formability, bending formability, Ag plating property and soldering wettability.
2. Description of Related Art
As lead frames for use in semiconductors, ferreous materials represented by 42 alloys and cupreous materials such as Cuxe2x80x94Nixe2x80x94Si series alloys, Cuxe2x80x94Sn series alloys, Cuxe2x80x94Cr series alloys, Cuxe2x80x94Fexe2x80x94P series alloys have been used so far. The cupreous materials have higher conductivity compared with ferreous materials and, accordingly, have an advantageous feature of excellent heat dissipation. Further, since the recent trend of using Pd (palladium) for exterior plating of IC or LSI results in a problem of peeling due to aging deterioration of the plating in the ferreous materials, the cupreous materials has been used more and more. On the contrary, since the cupreous materials has low strength, various improvements have been made for enhancing the composition or in the manufacturing method for increasing strength. This was considered extremely important, particularly, in the past stage where LSI packages using lead frames represented by QFP (Quad Flat Package) in which the number of leads exceeds 200 pin were developed vigorously.
In recent years, area mounted type packages represented by BGA (Ball Grid Array) have been developed and most of LSIs exceeding 200 pin have now been replaced progressively with such packages. However, such area mounted type packages are not suitable in a situation where the heat generation amount of semiconductor chips is increasing along with increase in the degree of integration and operation speed of LSIs. Therefore, it is necessary to attach heat dissipating plates or heat spreaders for enhancing the heat dissipation which makes the packaging complicated.
As described above, a reasonable heat dissipation method is one of subjects in packages mounting chips of large heat generation amount and packages using the former lead frames have now been re-estimated. In the packages using the lead frames, most of heat is dissipated by way of paths the leads to the substrate.
In this case, high heat conductivity due to the material of the lead per se has an effect on the heat dissipation of the entire packaging. Since the heat conductivity is in a linear relationship with the electroconductivity, a material of high electroconductivity is demanded in other words. In this regard, the ferreous 42 alloy has an electroconductivity as low as 3% IACS but the cupreous materials have higher electroconductivity and are advantageous.
Accordingly, a cupreous material having not only general characteristic as the lead material but also strength comparable with that of 42 alloy is demanded. Thus, copper alloys such as Cuxe2x80x94Nixe2x80x94Si series or Cuxe2x80x94Sn series alloys capable of providing high strength, or Cuxe2x80x94Cr series or Cuxe2x80x94Fexe2x80x94P series alloys capable of providing high electroconductivity have been used.
As the method of overcoming such problems, copper alloys of high strength and high electroconductivity by improving Cuxe2x80x94Fexe2x80x94P series alloys have been proposed, for example, in JP-A-Nos. 298679/1998, 298680/1998 and 199952/1999.
Since any of the alloys described above contains 0.5% or 0.3% or more of Fe and 0.1% or more of P, so-called internal oxidation tends to occur frequently upon heat treatment. The oxide layers extremely deteriorate the soldering wettability even when they are formed by such a slight thickness as can not be measured by instrumental analysis. In addition, since Mg is incorporated by 0.05% or more in JP-A-No. 199952/1999, it may be a worry of abnormal precipitation in Ag plating (hereinafter referred to as Ag plating protrusion).
A copper alloy as disclosed in JP-A-No. 54043/2000 has been proposed intending for high strength and high electroconductivity by incorporation of Ni, Fe and P. However, no consideration is made there on the softening resistance.
In view of the above, this invention intends to provide a copper alloy of high strength and high electroconductivity which is excellent in characteristics such as strength, electroconductivity and bending formability required as copper alloys for use in electric and electronic parts such as lead frames, terminals and connectors, as well as excellent in the characteristics such as softening resistance, shearing formability, plating property and soldering wettability by overcoming the foregoing problems.
A copper alloy for use in electric and electronic parts according to this invention comprises:
Ni: 0.1 to 1.0 mass %
Fe: 0.01 to 0.3 mass %
P: 0.03 to 0.2 mass %
Zn: 0.01 to 1.5 mass %
Si: 0.01 mass % or less and
Mg: 0.001 mass % or less, wherein
the relation for the Ni content, Fe content, P content and Si content satisfies the following relations simultaneously:
P content/Si contentxe2x89xa710
5xe2x89xa6(Ni content+Fe content)/P contentxe2x89xa67
4xe2x89xa6Ni content/Fe contentxe2x89xa69.
In the copper alloy described above, it is preferred to precipitate precipitates of Ni/Fe/P of (0.5 to 5)/(0.1 to 2)/1 at the mass ratio.
The copper alloy may comprises one or both of {circumflex over (1)} one or more of Co, Cr and Mn by 0.005 to 0.05% in total and {circumflex over (2)} one or more of Al, Sn, Zr, In, Ti, B, Ag and Be by 0.005 to 0.05% in total. Copper alloys containing the elements described above by less than the lower limit as inevitable impurity can of course be included in this invention.
It is preferred to restrict O: 100 ppm or less and H: 5 ppm or less among in the inevitable impurities.
The reasons for restricting the ingredients and conditions as described above are to be explained.
[Ni Content]
Ni precipitates an intermetallic compound together with P to be described later to enhance the strength of a copper alloy. Since the NiP compound is an not intermetallic compound stable at high temperature, it is poor in the softening resistance. However, the softening resistance is outstandingly improved while keeping the strength as it is by the incorporation of Fe to the Nixe2x80x94P precipitates to form a ternary intermetallic compound. In addition, the shearing formability is also improved.
When the Ni content is less than 0.1%, since the precipitation amount of the intermetallic compound is small, desired high strength and shearing formability can not be obtained. On the other hand, when the Ni content exceeds 1.0%, a great amount of coarse precipitates of the Nixe2x80x94P compound is formed during casting to extremely deteriorate the hot formability. The Nixe2x80x94P compound deteriorates the hot formability particularly in a temperature region of 700 to 900xc2x0 C. This temperature range is most required practically since hot working at high working rate is possible with a low energy because of the low transformation resistance. Further, even when the hot fabrication or working is possible below this temperature region, the remaining NiP compound scarcely contributes to the improvement of the strength and deteriorates the bending formability of products.
Accordingly, the Ni content is defined as 0.1 to 1.0%. A more preferred range is from 0.3 to 0.7%.
[Fe Content]
Fe causes both high strength and high softening resistance for the copper alloy by forming an intermetallic compound with Ni and P as described above. When the Fe content is less than 0.01%, the Nixe2x80x94P compound can not be transformed into an Nixe2x80x94Fexe2x80x94P ternary compound and the copper alloy can not effectively satisfy the demand for high softening resistance required for lead frames, terminals and connectors. For coping with the recent requirement for reduction of thickness and size and improvement for the mounting density in various kinds of electric and electronic equipments, a technique of decreasing the residual stress generated by shearing upon press punching has been developed and used generally. This is a technique of applying a heat treatment once for a short period of time from several seconds to several minutes upon lead punching while bundling the leads as they are without cutting off the top ends thereof thereby relieving the residual stress caused upon punching the lateral sides of the leads, subsequently cutting off the top ends of the leads to ensure flatness. When the softening resistance of the copper alloy is low, the material is softened during the heat treatment in the short period of time to cause deformation of frames upon cutting off the lead top ends. Even when the frame could be worked, disadvantageous such as frame deformation occurs during subsequent assembling of LSI.
In addition, Fe also has an effect of improving the hot formability in a copper alloy to which Ni and P are added. As described above, Ni tends to form coarse precipitates of Nixe2x80x94P compound upon casting and the precipitates which extremely deteriorate the hot formability in a range of 700 to 900xc2x0 C. In this case, Fe, being transformed into the Fexe2x80x94P compound, provides an effect of suppressing the generation amount of precipitates and improving the hot formability of the Nixe2x80x94P compound.
On the other hand, when the Fe content exceeds 0.3%, Fexe2x80x94P compound precipitates predominantly to the precipitation of Nixe2x80x94Fexe2x80x94P compound. As a result, not only the high strength and high softening resistance obtained by the precipitation of the Nixe2x80x94Fexe2x80x94P compound can not be obtained but also the shearing formability (press punching performance) is not improved.
Further, Fe is most likely to form internal oxide layers upon annealing next to element such as Mg or Si. When a heat treatment is applied in a low oxygen atmosphere in order to suppress external oxidation of Cu, growth of the internal oxide layer is more promoted than that in atmospheric air. Further, since it proceeds from the surface of the matrix material into the inside of the bulk, the oxide layer once grown can not but be removed by etching the surface of the matrix using, for example, a mixed solution of sulfuric acid and hydrogen peroxide. Thus, the growth of the oxide layer deteriorates pickling property. Then, when the oxide layer remains even little, it gives undesired effect on the surface property such as defective gloss in Ag plating or deterioration of the soldering wettability. As described above, while short time annealing is adopted generally with an aim of removing residual stress formed upon lead punching as described above, the heat treatment is applied by using a tunnel or the like and the atmosphere therein is a low oxygen atmosphere that promotes internal oxidation. The internal oxidation tends to be caused remarkably when Fe exceeds 0.3%.
Accordingly, the Fe content is defined as 0.01 to 0.3%. A more preferred range is from 0.05 to 0.2%.
[P Content]
P forms an intermetallic compound with Ni and Fe, which precipitates in the Cu matrix phase to improve the strength and the softening resistance of the copper alloy. Further, it forms precipitates different from Nixe2x80x94Fexe2x80x94P precipitates together with Co, Cr, Mn to be described later to give an effect of improving the shearing formability. However, when the P content is less than 0.03%, the precipitation amount of the Nixe2x80x94Fexe2x80x94P precipitates is not sufficient to obtain desired strength and softening resistance. Further, when the P content exceeds 0.2%, a great amount of precipitates of the Nixe2x80x94P compound described above is formed to extremely deteriorate the hot formability.
Accordingly, the P content is defined as 0.03 to 0.2%. A more preferred range is from 0.06 to 0.15%.
[Zn Content]
Zn has an effect of reducing the wear of a pressing mold and preventing migration and improves the heat resistant peeling property of solder and Sn plating. When the Zn content is less than 0.01%, no desired effect can be obtained. On the other hand, when the content exceeds 1.5%, the electroconductivity is lowered and the soldering wettability is also deteriorated.
Accordingly, the Zn content is defined as 0.01 to 1.5%. A more preferred range is 0.05 to 0.5% and a further preferred range is 0.05 to 0.2%.
[Si Content]
Si is chemically bonded with Ni to form an intermetallic compound Ni2Si, which precipitates in the alloy. However, no sufficient precipitation can be formed unless the temperature is higher than the temperature region where the Nixe2x80x94Fexe2x80x94P compound described above is precipitated. Accordingly, it is difficult that Si forms the Nixe2x80x94Si compound under the heat treatment condition optimized to the precipitation of the Nixe2x80x94Fexe2x80x94P compound. As a result, since most of Si is solid-solubilized in the matrix material of the alloy, not only the electroconductivity is lowered, but also the heat resistant peeling property of soldering and Sn plating is deteriorated when the relation with the P content to be described later is not satisfied. Further, Si is an element tending to cause internal oxidation like Fe described above and solid solubilized Si greatly promotes internal oxidation and also deteriorates the bending formability. Such effects become conspicuous when the Si content exceeds 0.01%.
Accordingly, the Si content is restricted as 0.01% or less (including 0%). A more preferred range is 0.005% or less.
[Mg Content]
Mg forms a compound with S inevitably intruding into the matrix material to form an Mgxe2x80x94S compound thereby deteriorating the Ag plating property. When the compound is present, abnormal precipitation occurs upon Ag plating to cause Ag protrusion. When an Si chip is bonded while leaving the protrusion as formed, localized stress is applied to the protrusion to cause chip cracking. Further, Mg tends to cause internal oxidation like Fe or Si and also to deteriorate the bending formability. These effects become conspicuous when the Mg content exceeds 0.001%.
Accordingly, the Mg content is restricted to 0.001% or less. A more preferred range is 0.0005% or less.
[P Content/Si Content]
The relation between the P content and the Si content concerns formation of the intermetallic compound with Ni. The heat resistant peeling property of soldering and Sn plating is deteriorated as described above, depending on the relation with the P content. When the value for the P content/Si content is less than 10, since the amount of solid-solubilized Si increases, the heat resistant peeling property of the solder and the Sn plating is undesirably deteriorated remarkably.
Accordingly, the relation between the P content and the Si content is defined as: P content/Si contentxe2x89xa710. A more preferred range is: P content/Si contentxe2x89xa715.
[(Ni Content+Fe Content)/P Content]
[Ni Content/Fe Content]
When the Ni content, the Fe content and the P content simultaneously satisfy the relations: 5xe2x89xa6(Ni content+Fe content)/P contentxe2x89xa67 and 4xe2x89xa6Ni content/Fe contentxe2x89xa69, the strength and the softening resistance are improved remarkably. That is, when the two relations are satisfied, the Nixe2x80x94Fexe2x80x94P compound is precipitated within a more preferred range of the compositional ratio to be described later. When the precipitates are precipitated finely and uniformly, the strength can be improved by precipitation hardening and since it has stability at high temperature, different from the Nixe2x80x94P compound, softening resistance is excellent.
Accordingly, it is preferred that the Ni content, Fe content and P content satisfy the two relations described above. A more preferred range is: 5xe2x89xa6(Ni content+Fe content)/P contentxe2x89xa66, and 4xe2x89xa6Ni content/Fe contentxe2x89xa68.
[Compositional Ratio for Ni/Fe/P]
As described above, the composition of the precipitate changes depending on the relation for the Ni content, Fe content and P content and high strength. High softening resistance can be attained simultaneously when the compositional (mass) ratio of Ni/Fe/P is: (0.5 to 5)/(0.1 to 2)/1. Accordingly, it is preferred that the precipitates of the Ni/Fe/P compositional ratio within the range described above are precipitated. A more preferred range is: (2 to 5)/(0.5 to 1)/1.
[Co, Cr, Mn Content]
Co, Cr and Mn form a compound with P to precipitate in the copper alloy and improve the shearing formability. When the compound is dispersed in the copper alloy, metallurgical continuity with the matrix material is tended to be interrupted because the precipitating behavior is different from that of the Nixe2x80x94Fexe2x80x94P precipitate described above (relatively large precipitates are formed), thereby enabling to improve the sharing formability remarkability. This effect is shown remarkably when the total content of Co, Cr and Mn is 0.005 or more.
However, this compound tends to form not uniform precipitates compared with the Nixe2x80x94Fexe2x80x94P compound. Particularly, since it precipitates preferentially at the crystal grain boundary, micro structures tend to be grown not uniformly to deteriorate the bending formability. This phenomenon appears remarkably when the total content of Co, Cr and Mn exceeds 0.05%.
Accordingly, when they are added, the total content of Co, Cr and Mg is defined as 0.005 to 0.05%.
 less than Al, Sn, Zr, In, Ti, B, Ag, Be Content greater than 
As described above, a technique of decreasing the residual stress formed by shearing upon press punching has been developed and adopted generally. In this technique, it is necessary that the material per se has high softening resistance so as not to be softened by annealing in the course of the punching process. The elements described above improve the strength by solid solubilization into the copper alloy and, further, provide more excellent softening resistance for the copper alloy in a state coexistent with the Nixe2x80x94Fexe2x80x94P precipitates.
For removing the residual stress formed by shearing upon press punching, it is necessary to heat the material so that dislocations in the material can be displaced easily. The residual stress is removed by the movement of the dislocations. However, when the dislocations are displaced, the dislocations cause pair extinction to lower the dislocations density. In other words, work-hardened material is softened by the movement of the dislocations. In this case, when the elements described above are solid solubilized, the atoms have high affinity with vacancies to bury the vacancy sites with the atoms. Therefore, the amount of vacancies in the alloy is decreased to suppress the upward movement of the dislocations, and the dislocations trapped in the Nixe2x80x94Fexe2x80x94P precipitate tend to move less easily. As a result, pair extinction of the dislocations are suppressed to increase the softening resistance of the copper alloy.
This effect is not sufficient when the total content of the elements described above is less than 0.005%, whereas the electroconductivity is lowered and the soldering wettability is deteriorated when it exceeds 0.05%. Accordingly, the content of the elements is defined as 0.005 to 0.05% as one or the total of two or more of them.
 less than O Content greater than 
O tends to easily react with P. When O exceeds 100 ppm, the reacted P can no more form a compound with Co, Cr and Mn described above. As a result, this can not provide the effect of improving the shearing formability. In addition, the soldering wettability is also deteriorated.
Accordingly, the O content is 100 ppm or less, more preferably, 40 ppm or less and, further preferably, 20 ppm or less.
 less than H Content greater than 
When O is contained by 100 ppm or more as described above, H is bonded with O into steams in the cooling process of casting when the H content exceeds 10 ppm, and the steams cause blow hole defects in cast ingots. As a result, internal defects referred to as overlapped surface or swelling is caused during heat treatment in the products.
Accordingly, the H content is 10 ppm or less, more preferably, 4 ppm or less and, further preferably, 2 ppm or less.